Lead author and Kaiser research scientist Rachel Whitmer says participants were initially categorized as either normal body weight, overweight or obese. "And even in the people who are in that normal body mass category, people who are not overweight or obese as defined by body mass index, if they had a large belly they had a 90 percent greater risk of dementia."

Whitmer says the effect of the large belly seemed to go up with weight. "The people who were both obese and with a large belly had a 260% greater risk of dementia."

While the study does not explain why belly fat targets the human brain, Whitmer says animal studies show that visceral fat or the fat wrapped around the inner organs may provide some clue. "And we do know from other studies that this fat, this very active visceral fat, plays a role in insulin resistance and atherosclerosis."

Whitmer says the good news is that visceral fat is the easiest to lose. "So when people do make an effort to diet and exercise and lose weight the fat actually comes off from that area first." Whitmer adds that fat around the belly, unlike family history or age, which are risk factors for Alzheimer's disease that are not modifiable, "This a risk factor that is modifiable."

The next step researchers say is to study whether the substances secreted from the fat are associated with memory and cognitive functions. They also want to know if loosing that belly fat lowers the risk of dementia. Whitmer says while dementia may appear later in life, it is never too early to think about risk factors for the disease, especially those you can control.